Lift of an airplane wing is a function of its forward speed and angle of attack. An airfoil such as an aircraft wing develops a lift force as a result of air movement across generally opposite sides or surfaces of an airfoil at unequal velocities. In the typical case of an aircraft wing, for example, air moves across the upper wing surface at a greater velocity than across the lower wing surface, thus producing a differential pressure on the wing which generates a net upward lift force thereon. The amount of differential pressure and consequently the amount of lift developed by a wing generally is dependent upon the difference in velocities of air movement across the opposed wing surfaces, and the relatively greater velocity of air movement across a wing upper surface relative to the wing lower surface commonly is obtained by appropriately forming the camber of the wing upper surface with respect to the lower surface.
Under certain conditions of aircraft operation, the relatively streamlined airflow across the upper surface of the wing tends to become partially or substantially completely separated from the wing upper surface. This flow separation typically occurs with an aircraft wing at relatively low flying speeds found, for example, at landing or takeoff conditions when the wing is being operated at a relatively high angle of attack and when maximum lift generation is particularly critical. In many aerodynamic components the flow detachment originates at the trailing edge of the airfoil section and expands both forwardly and in a spanwise direction as the angle of attack is increased.
In an attempt to remedy this problem, vortex generator devices are used to delay or eliminate flow separation along a flow control surface. Vortex generators on aircraft are used to mix high-energy air outside of the boundary layer with the retarded air within the boundary layer. This allows the boundary layer to remain attached longer in regions of the flow with adverse pressure gradients. Following their initial use, a number of experimental studies indicated that if vortex generators were incorporated properly, they could be used to improve lifting effectiveness, extend the point of buffet onset, improve lift-to-drag ratio, or enhance stability.
An example of a typical vortex generator is the conventional vane type vortex generators used widely on aircraft for lift enhancement and drag reduction. Such a device may be found by reference to U.S. Pat. No. 5,253,828 to Cox. Vane type vortex generators are sharp blades extending normal to the surface and with an angle of attack to the flow at least during the time vortices are to be generated. Flow migrating over the outer tip of each vane can create a useful streamwise vortex that mixes high speed fluid from the free stream into the sluggish energy-deficient boundary layer. This mixing increases the energy in the boundary layer, making it resistant to flow separation. The process is called passive boundary layer control.
It is also common practice to place vortex generators at certain selected locations on an upper surface of an airfoil to extend the onset of flow separation. An example of such a device may be found by reference to U.S. Pat. No. 4,323,209 to Thompson. By creating the vortex, the high momentum fluid particles outside the boundary layer are mixed with the retarded boundary layer air at the surface, thus avoiding or extending the occurrence of separated flow.
Essentially, vortex lift control involves stabilizing the vortex shed from the leading edge of the wing so as to lock the leading edge vorticity along the spanwise direction of the wing. This causes lift-producing, stream-line airflow to pass over the upper surface of the wing, over the locked vortex, and then to become reattached to the wing surface. This results in an effective increase of the wing camber and thus increased lift.
However, many of the prior art devices are deficient in light of the present device. For instances, many of the designs create substantial drag that outweighs any beneficial lifting affect created by the device. Additionally, many of the previous designs fail to produce sufficient lift to justify their increased costs.
It is readily apparent that a new, improved and relatively inexpensive leading edge lifting vortex controller is needed that is attached to the leading edge of an airfoil in substantially the same plane as the airfoil and that provides significant aerodynamic improvements yet substantially limits detrimental drag affects. It is, therefore, to the provision of such an improvement that the present invention is directed.